1. Field of the Invention
This invention relates to a process for the surface treatment of articles having iron or iron alloy surfaces, more particularly to an improved process for sulfuring treatment of ferrous articles to improve their seizure-resistance property, wear-resistance property and the like by electrolyzing their ferrous surfaces to form FeS-system compounds thereon.
2. Description of the Related Art
When a ferrous article is electrolyzed as an anode in a molten salt bath of potassium thiocyanate and sodium thiocyanate, its ferrous surface is formed with a sulfurized layer (FeS-system compound layer) which markedly enhances the seizure-resistance property and wear-resistance property of the article. Sulfurizing technologies of this type are taught, for example, by JPB-44-1809, JPB-63-12158, and JPA-6-220689.
As set out in these publications, the molten salt used consists of, for example, potassium thiocyanate (rhodan potassium) and sodium thiocyanate (rhodan sodium) at a ratio of about 3:1 and the electrolysis is considered to be preferably effected at a temperature of about 190.degree. C..+-.5 .degree. C. and a current density of around 1.5-4.0 A/dm.sub.2.
This sulfuring treatment is characterized by its ability to improve the surface condition of various machine elements (components) subject to friction. The treatment is therefore most frequently applied to small components having precision machined dimensions such as small gears.
Since these machined components are used as rotating members or sliding members, the sections thereof lying perpendicular to the center axis are generally symmetrical. However, their surfaces include hills and valleys along the surface configuration. The worth of the component is determined by the dimensional precision of these irregular configurations and the uniformity of the surface quality. It is therefore essential that the component treated by sulfurizing incur no change in shape or dimensions relative to those prior to the treatment and that its surface be formed with a uniform sulfurized layer.
When such small articles are subjected to sulfuring treatment in an electrolysis crucible, however, an attempt to treat them all under exactly the same electrolysis conditions encounters various difficulties. For example, processing the individual articles as anodes makes it impossible to effect continuous treatment in the manner used to plate steel strip. Batch treatment therefore has to be adopted. In this case, each article has to be suspended in the bath as an anode. When an attempt is made to increase productivity by electrolyzing a few to ten articles simultaneously (in batch electrolysis), the position in the bath (the anode position, i.e., reaction position) differs between the different articles. As a result, the current density and, accordingly, the reaction rate differ from one article to another. Since this gives rise to differences in reaction behavior caused by uneven temperature distribution and composition distribution of the bath, it is difficult to stably form uniform sulfurized layers at the surfaces of all of the articles.
Further, when the articles are placed in the bath, matter adhering to the article surfaces mixes into and fouls the bath, while the sensible heat of the articles also produces changes in the bath. In addition, the treated articles hoisted out of the bath have bath components adhering thereto, so that the bath level (volume) is changed. Since these changes in the bath temperature, bath composition and bath volume express themselves as changes in the electrolysis conditions between one batch and another, they thwart the attempt to form uniform sulfurized layers in all batches.